1. Field of the Invention
The present invention relates to a coating having excellent oxidation resistance and corrosion resistance, which is provided on the surface of metals such as molybdenum, niobium and their alloys, and manufacturing method thereof.
2. Description of the Background Art
Due to properties of low vapor pressure and thermal expansion coefficient, Molybdenum (Mo) having a melting point of 2617° C. maintains high strength and hardness at a high temperature, and has better high-temperature mechanical, thermal properties than any other metal. Accordingly, it is a core material which can be applied to fields of aerospace, atomic energy and the like.
However, the material has a disadvantage that it can be used only in a non-oxidizing condition since it forms volatile MoO3 by reacting with oxygen at a low temperature of about 600° C.
On the other hand, since niobium (Nb) has a melting point of 2467° C., a density lower than that of molybdenum (Nb; 8.55 g/cm3, Mo; 10.2 g/cm3), and its high-temperature mechanical property is excellent as that of molybdenum, niobium or niobium alloys can be advantageously used as next-generation high-temperature structural materials. However, these materials also do not show high-temperature oxidation resistance.
To improve oxidation resistance of molybdenum or molybdenum alloys, methods such as surface coating treatment, by which MoSi2 having an excellent oxidation resistance is coated on a molybdenum surface, have been widely used. In case of niobium or niobium alloys, to improve oxidation resistance, a surface coating treatment, which is similar to that for molybdenum and is performed after depositing molybdenum on the surface in a predetermined thickness, has been being studied.
In case of a slurry surfacing method among the surface coating treatments, the formation of an alloy coating can be easily performed, but a amount of defects, such as pore and the like can be formed.
In case of directly coating a MoSi2 layer using a low-pressure plasma spraying method, the formation of the alloy coating is easily performed, but it is difficult to adjust the composition and to form MoSi2 coating without a defect.
Therefore, reactive-diffusion methods such as pack-siliconizing, CVD, dipping of liquid silicon and the like are generally adopted as a coating treatment. The pack-siliconizing method and CVD method, in which the silicon diffuse under the condition of gas state and are deposited on the surface of the basic materials, are distinguished from the dipping method in which the silicon diffuse under the liquid condition.
Since a dense silicon dioxide (SiO2) layer is formed on the surface of MoSi2 layer and restrains movement of oxygen when the MoSi2 layer coated on molybdenum or niobium is exposed to a high-temperature oxidation atmosphere, internal base materials can be protected.
However, thermal, mechanical limitation which are problematic for commercialization of MoSi2 coating is affected by following three factors.                (1) interdiffusion between molybdenum or niobium and MoSi2 coating,        (2) thermal stress generated by a difference of thermal expansion coefficients between molybdenum (5.1×10−6/° C.) or niobium (7.2×10−6/° C.) and MoSi2 coating (9.5×10−6/° C.), or between a difference of thermal expansion coefficients between MoSi2 coating and SiO2 layer (0.5×10−6/° C.), and        (3) pest oxidation that the MoSi2 coating is divided into MoO3 and SiO2, which is due to the rapid oxidation occurred in the atmosphere around 400˜600° C. and accordingly,        
Therefore, in case of actually using molybdenum or niobium on which the MoSi2 coating is coated, a life span of the coating varies according to the condition under which it is used.
In case of isothermal oxidation which occurs in a high temperature oxidation atmosphere, silicon is diffused into molybdenum by the interdiffusion between molybdenum and MoSi2 coating, and, accordingly, the MoSi2 coating with excellent oxidation resistance is transformed into a Mo5Si3 coating without oxidation resistance, which can not be used as a surface protecting coating anymore. Therefore, in this case, the maximum life span of MoSi2 coating can be increased by increasing the thickness of it.
However, in case of cyclic oxidation occurring during the cyclic process of maintaining the above material in a high temperature oxidation atmosphere for a predetermined time and then cooling to a room temperature, when the temperature is raised to a high temperature, the micro crack in the coating is filled with silicon oxides formed from silicon within MoSi2 coating (self-healing). On the other hand, when the coating is cooled to the room temperature, micro cracks are generated within the coating, due to the difference of thermal expansion coefficients between molybdenum or niobium and MoSi2 coating and silicon oxide layers.
As the number of cyclic oxidation between high temperature and room temperature increases, the size of the micro crack increases. When the size reaches to a critical point, the crack can not be filled any more, and molybdenum or niobium is directly exposed to oxygen which exists in the atmosphere, thus causing rapid oxidation.
In addition, the other problem is that, in the atmosphere around 400˜600° C., the MoSi2 coating is rapidly oxidized into the powder types of molybdenum oxides (MoOx) and silicon oxides. As described above, this kind of oxidation is called as pest oxidation.
Particularly, volume expansion of about 250%, which is occurred when MoSi2 is oxidized at a low temperature into molybdenum oxides and silicon oxides, causes generation of a pore and a micro crack, and disintegration of the MoSi2 coating into a powder type. Accordingly, the MoSi2 coating get lost low-temperature oxidation resistance.
Therefore, the reason that commercialization of molybdenum or niobium coated with MoSi2 is difficult is that it has no cyclic oxidation resistance at a high temperature and no low-temperature oxidation resistance.
To improve cyclic oxidation resistance and low-temperature oxidation resistance of molybdenum or niobium coated with a MoSi2 layer, two conventional methods have been developed.
Firstly, there is a method for improving cyclic oxidation resistance by filling the crack. In this method, when an alloy element is added to the MoSi2 coating, a silicon oxide layer which is formed in a high-temperature oxidation atmosphere is alloyed to reduce a difference in thermal expansion coefficient between MoSi2 coating and silicon oxide layer. Accordingly, at the room temperature, the peeling of oxide layer is restrained, and the viscosity of silicon oxide layer is reduced, and thus the oxide layer smoothly slips down into the micro crack.
As an example of the above method, the U.S. Pat. No. 2,865,088 disclosed that the cyclic oxidation resistance could be improved by the addition of chrome (Cr), boron (B) and the like. And the German Patent No. 1,960,836 has reported that the cyclic oxidation resistance was improved about five times in case of adding germanium (Ge).
Secondly, according to B. V. Cockeram et al. reported in the Oxidation of Metals, vol. 45 (1996) p. 77˜108, if sodium fluoride (NaF) is used as an activator in a manufacturing process of a MoSi2 coating by a pack-siliconizing method, the sodium fluoride deposited on a surface of the coating layer has been known to be capable of improving low-temperature oxidation resistance of coating.
On the other hand, according to the disclosure of U.S. Pat. No. 5,472,487, when a properly mixed powder of MoSi2, SiO2, Si3N4, SiC and Mo5Si3 is coated by low-pressure plasma spraying on a niobium (Nb) metal having a thermal expansion coefficient of 7.9×10−6/° C., the thermal expansion coefficient of a composite coating becomes lower than the thermal expansion coefficient of the pure MoSi2 coating, and the peeling of the surface protecting oxidation coating or the composite coating are not observed even if cyclic oxidation tests in which the metal is heated for an hour in an oxidation atmosphere of 2500° F. (about 1371° C.) and then is maintained for 55 minutes in room temperature, are cycled about 10 or more times.
However, the devices of the low-pressure plasma spraying method cost very much, a plurality of defects such as pores and the like exist in the coating, and the method is limited to be used in manufacturing of a thick coating having a thickness of several mms.
On the other hand, methods for reducing thermal expansion coefficient of MoSi2 sintered composite improving pest oxidation are also reported.
The U.S. Pat. No. 6,288,000 discloses that a thermal expansion coefficient of MoSi2—Si3N4 sintered composite which was manufactured by hot-isostatic pressing of powder mixture formed by adding MoSi2 powder and Si3N4 powder having volume ratios of about 30% and 50% is lower than the thermal expansion coefficient of monolithic MoSi2 and is similar as that of Mo at 1000˜1500° C.
The U.S. Pat. No. 5,429,997 disclosed that low-temperature oxidation resistance (in the atmosphere at 500° C.), high temperature isothermal oxidation resistance and cyclic oxidation resistance of MoSi2—Si3N4 sintering which was manufactured by hot-isostatic pressing of powder mixture in which respectively Si3N4 powder having 30% and 45% of volume ratios is added is more excellent than in the case of monolithic MoSi2.
FIG. 1 is a view showing a cross-sectional microstructure of MoSi2—Si3N4 sintered composite manufactured by hot-isostatic pressing in the Materials Science and Engineering A261 (1999) p. 24-37 reported by Mohan G. Hebsur.
As shown in FIG. 1, the microstructure of the sintered composite is characterized in that the Si3N4 particles are irregularly formed in the MoSi2 matrix. Therefore, the method was not efficient in preventing oxygen from diffusing through a MoSi2 grain boundary into the layer by forming a Si2ON2 protection layer.